1. Technical Field
The present disclosure relates to a distributed control system. More particularly, the present disclosure relates to a distributed control system in which a plurality of field controllers are connected through a control network.
2. Related Art
In a distributed control system that controls field devices in a plant through field controllers disposed in the plant in a distributed manner, a plurality of devices such as an engineering station, an operation monitor and field controllers are connected to each other through a control network. Among a plurality of field controllers, the process data of the other field controllers can be received by way of the control network and the received data can be used for their own control computations. Such a control computation (control scan) is executed in a given cycle at each field controller (see e.g., JP-A-2006-276958).
The control computation of the field controller is started once the power is turned on. For this reason, when a plurality of field controllers are present, the timing of the control computation of each field controller depends on the timing of power-on.
If the timing of the control computation can be made the same among the field controllers, the overall computation speed can be increased and stable control can be performed. Therefore, it is considered to execute the control computations of all the field controllers in synchronism with one another in accordance with the network time. However, at the same time, in the control computations of the field controllers, continuity of each control computation is also required. For example, when a momentary power failure occurs at a specific field controller, it is necessary to resume the control computation from where it is halted, after the power is restored. However, when the control computation is executed in accordance with the network time, such continuity of the control computation is lost.